Currently, communication systems around the world are growing rapidly due to the increasing need for data communication bandwidth. In particular, satellite communication systems are currently experiencing rapid growth due to growing customer demand for satellite based data communications. Most of the current demand and estimated future demand will be for Internet and other network based data communication applications. A major factor in these communication systems is the bandwidth capacity demanded by the user. Applications in widespread use today, such as video conferencing, LAN/WAN and document delivery require high speed forward and return link data capacities. Currently, however, these capabilities are not provided by present day Internet via satellite systems.
It is currently estimated that there are approximately 13 million hosts and 16 million users on the Internet. The growth rate has been approximately 10 million new users a year for the past four years. At the same time, the number of Intranets (Internet network protocols applied within an enterprise or company for sharing information) are growing at an even faster rate. Currently, accessing the Internet via satellite has gained recognition as a fast and reliable solution for fast Internet access. Current commercially available Internet via satellite solutions such as DirecPC are based on an asymmetrical approach in which the data link to the user is via satellite while the return link to the user is via telephone lines using commercially available telephony modems. The disadvantages of these asymmetrical systems is outlined below.
The asymmetric approach via satellite, in which the user receives data from the Internet via satellite, yet sends data to the Internet via telephone lines, does not take advantage of a major feature of satellite communications: wide area coverage. The asymmetric link is based on a terrestrial connection and therefore limits the ability of the fast connection to the Internet to those places in which telephone lines and Internet service providers are available and have sufficient grade of service.
The data rate of asymmetric Internet via satellite communication systems enables basically e-mail and browsing applications only. This structure is mainly targeted to consumer markets where the user is limited to sending data from their home at relatively low speeds. There are, however, many users such as small office/home office (SOHO) that desire high speed data communications in both directions yet cannot afford having dedicated lease lines for their Internet connection. In the United States alone there are approximately 3.5 million small businesses of which only 10% can justify an expensive leased line. Thus, there are a large group of users looking for an on demand economical, fast and reliable connection to the Internet with a grade of service similar to that of a leased line.
Typical applications that require high data rates in both directions include video conferencing, LAN/WAN systems, Internet applications, document delivery, audio applications such as Internet Phone, commercial web sites, net gaming, point of presence, terminal equipment, Net Meeting and collaboration software. All the above mentioned applications are currently not adequately served by the currently available asymmetric satellite communication solutions.
Spread spectrum communication systems have been used in a variety of fields for some time now. In spread spectrum communication systems, the bandwidth of the transmitted signal is much greater than the bandwidth of the information to be transmitted. The carrier signal in such systems is modulated by a function that serves to widen or spread the bandwidth of the signal for transmission. On the receive side, signal is remapped or decoded into the original information bandwidth to reproduce the desired output signal.
Spread spectrum systems can be categorized into direct sequence systems, frequency hopping systems, time hopping systems and hybrid systems which are combinations of the above three.
In frequency hopping systems a carrier frequency is shifted or hopped in discrete increments in a pattern dictated by a predetermined code or sequence, e.g., a pseudo noise sequence or code. The resulting consecutive and time sequential frequency pattern is called a hopping pattern and the duration of each hop frequency is called a chip. The transmitted information is embedded in the codes or embedded within each frequency in the carrier wave by a modulation scheme such as PSK or FSK.
In reproducing the information signal of the receiver a synchronization acquisition process is performed in which the code pattern utilized by the receiver is synchronized with the code pattern generated and used in the transmitter. Using this, de-spreading and demodulation are performed on the spread spectrum signal to extract the transmitted data. A local reference signal is used that has a frequency corresponding to the same code pattern used in the transmitter for every chip. The received signal and the local reference are mixed in order to perform a correlation or de-spreading process for converting the spread spectrum signal into a signal having a frequency bandwidth wide enough to extract the data information. More information describing the operation of spread spectrum systems can be found in "Spread Spectrum Systems," by R. C. Dixon published by John Wiley and Sons, Inc., 1976.
Multiple user systems use multiple access techniques to allow users to share resources such as time and frequency. When the traffic from each user in the network is approximately steady it is possible to divide a single high capacity multiple access channel into a plurality of smaller orthogonal channels corresponding to individual user requirements. This can be accomplished either on a frequency basis using FDMA, on a time basis using TDMA or using CDMA. In addition, various combinations of FDMA and TDMA can also be used to minimize cost in large networks. FDMA and TDMA techniques are suitable solutions as long the traffic from each user is relatively stable. CDMA is a multiple access technique which uses spread spectrum communications. CDMA communications can be synchronous if all users are mutually synchronized in time.
TDMA communication systems are also known for providing multiple access. Theses systems partition the channel time in a fixed predetermined manner. They are efficient when the user population includes only a relatively small number of users having high duty cycles. However, many modern communication systems need to provide communication among interactive data terminals which operate in low duty cycle burst modes. Thus, TDMA is not particularly suited to this kind of communication.
In the typical modern interactive network, however, the traffic from individual terminals in the system varies as a function of time due to random traffic demands by different users at each terminal. In addition, the set of terminals active in the network can vary from moment to moment. In such systems it may be desirable to assign channel capacity to users on demand by means of a demand assigned multiple access (DAMA) architecture. In a DAMA system a separate channel called the request channel is used by individual users to request capacity as needed. The capacity can then be allocated in response to requests by a central master controller implemented by a common algorithm running in each terminal.
A DAMA system however introduces additional overhead into the multiple access channel due to the process of requesting and assigning capacity. In addition, the demand assignment process introduces a delay which can degrade the performance under the channel.
In some DAMA networks the total number of potential data terminals sharing the request channel is much larger than the number of terminals active at any given point in time. Thus, subdividing a DAMA request channel into smaller fixed allocation sub channels is impractical. It is thus necessary to design a request channel architecture based upon a random access technique which allows for the possibility of a small subset of active transmitters selected from a much larger set of potential transmitters. Two random access techniques are currently available for such applications which include ALOHA multiple access and CDMA.
The first data network to be based upon a random access protocol was ALOHANET which went into operation throughout the state of Hawaii in 1970. The ALOHA system was the first random access multi-point packet data communication system. The system uses a single radio channel shared by plurality of stations or data terminals. When a station generates a packet, the ALOHA system transmits the packet on the common radio channel. Since more than one station may attempt to transmit a packet at the same time several transmissions may overlap. The overlapping transmissions are said to collide if any portion of two transmissions overlap. When a collision occurs each station waits a random period of time before attempting to gain access to the channel again.
To increase the utilization of the radio channel, the slotted ALOHA system was proposed in which the channel is partitioned into time slots equal to a packet length wherein each station can only transmit a packet at the beginning of a slot. In this way overlapping transmissions are forced to completely overlap. Using a slotted approach almost doubles maximum channel utilization compared to the unslotted ALOHA system.
To reduce the effects of collisions in the slotted ALOHA system a slot reservation scheme was proposed. The channel was partitioned into frames each containing a reservation slot for transmitting a reservation packet and data slots for transmitting data packets. Each station transmits a reservation packet on a random access basis requesting slots needed for data packet transmission. If the request is granted data slots of a subsequent frame are assigned to the requesting station which subsequently transmits data packets on its assigned slots.
Satellite communications can provide point to point channels or broadcast and multiple access channels. A satellite is well suited to provide one to many i.e., broadcast, channels and many to one, i.e., multiple access, channels from and to an earth station. The architecture of the network used in very small aperture terminal (VSAT) data networks is almost always designed around a single large hub earth station transmitting data in a broadcast channel to a large number of VSATs as shown in FIG. 1. Considering Network A, for example, the VSATs 20 in such a network transmit data in packets to the hub station 18 using the multiple access capability of the satellite channel 17.
The communications from the hub station of a VSAT network to the VSATs themselves is easily configured using a conventional communication technique such as TDM or FDM. Currently, TDM is widely used for multiplexing the hub to the VSAT terminals, notwithstanding the fact that there are differences in data rate, modulation techniques and transmission formats among the various VSAT networks.
The multiple access link from the VSATs to the hub, however, is currently subjected to a greater degree of variation. The choice of multiple access technique from the VSATs to the hub is currently the primary feature distinguishing one network from another.
Recently, however, it has become commonplace to build VSAT networks composed of hundreds and thousands of more small VSAT terminals. The traffic in these networks is typically in the form of single data packets originating from interactive users or bursts of data packets originating from some type of file transcript protocol. As the number of stations in the network increases, the more the traffic from the single station will appear to fluctuate due to random user demand. In such networks, the use of FDMA or TDMA becomes impractical while the use of DAMA would impose an unreasonable amount of overhead in the network. Thus, to provide multiple access to these types of packet data networks, the access techniques of direct sequence (DS) spread spectrum multiple access, i.e., DS-CDMA and ALOHA, are used. Both these multiple access techniques however suffer from disadvantages. Direct sequence spread spectrum systems require the hub receiving station to have a digital matched filter operating at the high speed chip rate for each of the possible transmitters with each using a different spreading sequence. In a network with a large number of VSATs this becomes unwieldy and expensive to maintain. ALOHA systems suffer from relatively low capacity and high average power requirements.